1. Field of Invention
This invention relates generally to an apparatus for monitoring the wet bulb temperature in flue gas streams. Such monitoring is useful in conjunction with, for example, process control of a spray dryer absorption process and leak detection for boiler tubes within a coal-fired steam boiler.
2. Description of the Related Art
A coal-fired steam boiler produces superheated steam for use in the production of electricity by a turbine generator, and may produce steam for heating or other industrial use.
Two aspects of power generation are relevant background examples for the invention. The first of these aspects concerns detection of leaks in boiler tubes.
A power plant boiler has boiler tubes within which steam is generated. A problem which arises in power plants is leaking boiler cubes. As boiler tubes age through service they may deteriorate and a leak in a boiler tube may develop. When such a leak develops, steam escapes from within the boiler tube into a flue gas duct.
Leaks in boiler tubes result in the loss of thermal performance, and can cause corrosion, deposition, and fouling in downstream equipment. Tube leaks are major causes of power plant unavailability, and can result in a forced outage of the entire boiler system. Small leaks are often undetectable because any loss in performance is usually relatively small. Additionally, these leaks are often concealed by insulation, and are generally inaudible to standard acoustic monitoring equipment. Furthermore, small leaks can become self-compounding and intensify into additional, larger leaks. The water vapor released by these leaks combines with the flue gas impurities to produce acidic, highly corrosive conditions for other boiler tubes and downstream equipment. Measuring the wet bulb temperature within a flue gas duct can provide an indication of whether a boiler tube leak exists since the humidity of flue gas should increase in the event of a leak.
The second relevant background aspect of power generation is discussed in the following paragraphs.
The spray dryer absorption process for removing sulfur dioxide from flue gas utilizes an aqueous slurry of slaked lime (lime mixed with water) to chemically capture sulfur dioxide that is present in the flue gas. The resulting chemical reaction between the slurry and the gas stream leads to the formation of calcium sulfites and sulfates.
The absorber has a dryer vessel in which flue gas is mixed with an atomized slurry of lime (Ca(OH).sub.2) and recycled product (i.e., fly ash, CaSO.sub.4, CaSO.sub.3, CaCO.sub.3, and unreacted Ca(OH).sub.2). This mixing results in droplet drying and simultaneous sulfur dioxide removal yielding a spray dryer waste product consisting of fly ash mixed with calcium-sulfur compounds. Flue gas and most of the waste products then exit the dryer vessel and enter a fabric filter where the waste products are removed. Some of the waste products also drop out of the dryer vessel and are conveyed to a hopper for recycle to the process. Dry waste products collected by the fabric filter may either be recycled or conveyed to disposal. The cleaned flue gas stream which exits the fabric filter is discharged into the ambient air by way of stacks.
To maximize the capture of the sulfur dioxide from the flue gas the "approach to saturation temperature" is controlled. Saturation temperature, or wet bulb temperature, is defined as the lowest temperature at which a given amount of water vapor can be retained before droplets begin to condense. The approach to saturation temperature is defined in the spray drying art as the spray dryer exit temperature minus saturation temperature. Accordingly, both the dry bulb temperature (which is measured in the dryer vessel outlet gas) and the wet bulb temperature (which is measured in the dryer vessel inlet gas) need to be determined. Additionally, these two temperature measurements can be made at several locations both upstream and downstream of the dryer vessel.
For dry scrubbing applications, water vapor contained in the flue gas supplies the driving force for sulfur dioxide removal. The closer the approach to saturation temperature, the greater the removal for sulfur dioxide in the process, and the greater the efficiency of reagent use. Theoretically, the goal in this process would be to achieve identical temperatures for both the wet and dry bulb temperatures. In practice, however, operating in this mode would cause a condition known as "wet bottom" which consists of severe plugging of the spray dryer outlet ductwork due to the saturation by water of the reactive waste product leaving the dryer vessel. Therefore, this key operating parameter for the spray drying process requires that the resulting dry bulb temperature exceed the actual wet bulb temperature by approximately 20 degrees F. In the event that the approach to saturation temperature becomes more than approximately 20 degrees F., then the amount of reagent slurry sent to the spray dryer atomizer is decreased. This decrease in reagent slurry results in less cooling of the flue gas within the spray dryer, and a corresponding higher spray dryer outlet temperature due to a decrease in slurry water content. By measuring both the wet bulb and dry bulb temperatures, the approach to saturation temperature can be controlled through a feedback control loop to the atomizer feed pump supplying the slurry atomizer.
It is occasionally but not commonly a practice for a utility using spray drying technology to measure the wet bulb temperature of the inlet gas. Usually, the dryer vessel outlet gas temperature (dry bulb) will be fixed at a "safe" temperature (145-150 degrees F.) known to be much higher than the calculated saturation temperature. This methodology of control can result in reduced sulfur dioxide removal, and inefficient use of reagent since an unnecessarily large flow of lime reagent slurry is sent to the dryer vessel to insure that the desired level of sulfur dioxide is removed. The net effect is an overall decrease in the amount of system water sent to the dryer vessel. This decrease in system water causes the dryer vessel outlet gas temperature to become unnecessarily higher than that of the wet bulb temperature and thus, artificially raises the approach temperature. Furthermore, the lime reagent used in the spray drying process is costly so this conventional utilization results in increased reagent costs and increased reagent consumption due to the reduced utilization of recycled waste solids.
Attempts have been made to measure the wet bulb temperature in spray dryer systems. For example, a prior art wet bulb measuring apparatus is provided by U.S. Pat. No. 4,809,537, assigned to the assignee of the present invention. The '537 system diverts a sample of flue gas, filters the sample, and reheats the sample to the same temperature as the gas in the flue.
In a unit sensing the wet bulb temperature of a gas stream it is necessary that the surface of the measuring sensor remain wet without becoming clogged with particulates or excessively corroded. Furthermore, the sensor is preferably protected from the corrosive and erosive environments to which it is exposed while maintaining an accurate wet bulb reading. To date, these objectives have proven to be very difficult to attain with a reliable wet bulb temperature measuring instrument.